Photocatalytic efficiency under visible light of a novel Cu–Fe oxide composite films prepared by one-step sparking process

Copper–iron (Cu–Fe) oxide composite films were successfully deposited on quartz substrate by a facile sparking process. The nanoparticles were deposited on the substrate after sparking off the Fe and Cu tips with different ratios and were then annealed at different temperatures. The network particles were observed after annealing the film at 700 °C. Meanwhile, XRD, XPS and SAED patterns of the annealed films at 700 °C consisted of a mixed phase of CuO, γ-Fe2O3, CuFe2O4 and CuFe2O. The film with the lowest energy band gap (Eg) of 2.56 eV was observed after annealing at 700 °C. Interestingly, the optimum ratio and annealing temperature show the photocatalytic activity under visible light higher than 20% and 30% compare with the annealed TiO2 at 500 and 700 °C, respectively. This is a novel photocatalyst which can be replaced TiO2 for photocatalytic applications in the future.

www.nature.com/scientificreports/ were then deposited on the substrate with a deposition rate of 52.33 nm/min for 10 min after sparking off the Fe and Cu tips. The as-deposited films were then annealed at 500, 600, 700, 800, and 900 °C for 60 min to improve their crystallinity. Morphology, chemical and optical properties of the samples were characterized using scanning electron microscopy (SEM, JEOL JSM300 and SEM, JEOI JSM 6335F), transmission electron microscopy (TEM, JEOL JEM 2010), X-ray Photoelectron Spectroscopy (XPS, AXIS Ultra DLD-X-ray Photoelectron Spectrometer and a monochromatic AlKa X-ray excitation source) and UV-Vis spectroscopy (Hitachi U-4100).
Photocatalytic activity under visible light was investigated by the decomposition of methylene blue (MB) solution (Ajax Finechem). The samples were dipped into 3.0 mL of MB solution with a concentration of 10.0 µM and then irradiated a lamp (TB814SU-Y lamp with wavelength and luminance of 340-900 nm and 6.57 × 10 5 Lux, respectively) for 1-5 h. Degradation of MB can be indicated by measuring the absorbance using UV-Vis spectrophotometer.

Results and discussion
The mass loading of the TiO 2 and Cu-Fe composite photocatalyst on quartz substrates in this work was 0.12 mg/10 min. This result is in good agreement with our previous papers 26,27 . The effect of Cu:Fe ratio and annealing temperature on MB degradation have shown in Fig. 1a,b. Moreover, the annealed TiO 2 at 500 °C and 700 °C which were prepared by the sparking process were used to compare MB degradation with the Cu-Fe oxide film at 700 °C against irradiation time, as shown in Fig. 1c. It is noted that the annealed Cu-Fe oxide film at 700 °C with the ratio of 2:2 has a degradation performance higher than 20-30% compared with the wellknown photocatalyst such as TiO 2 . Thus, a new finding of Cu-Fe oxide which was used as photocatalyst is a strong point of this work. Figure 2a shows the morphology of the annealed Cu-Fe oxide film at 700 °C with a ratio of 2:2. The nanoparticles were aligned to networks with the length and width of 1410 nm and 279 nm. This is because of the high surface energy of nanoparticles, Cu-Fe oxide nanoclusters were agglomerated to decrease their surface energy. Meanwhile, the agglomeration of adjoining grains becomes more observable for higher kinetic energy 28,29 . According to the arrangement of the network particles, it can increase the MB decomposition which corresponds to Fig. 1b.
TEM image of the annealed Cu-Fe oxide film at 700 °C with a ratio of 2:2 is shown in Fig. 2b. It is clearly seen that the actual particle sizes are in the range of 4-15 nm, while the energy dispersive x-ray shows the amount of Cu, Fe, and O are 28.26, 31.44, and 40.35 atomic %, respectively (data not shown). Moreover, the selected area electron diffraction (SAED) (inset) shows well-established diffraction rings matching most closely with CuO in the (111) Fig. 2c. The mass loading of ~ 0.12 mg/10 min was obtained on the quartz substrate. The primary nanoparticles were agglomerated to form network particles. Figure 3a shows XRD patterns of the annealed Cu-Fe composite films at 700 °C with various Cu:Fe ratios. It is noted that the ratio has a direct effect on the quantity of each composite film. Interestingly, the ratio of 2:2 shows many Cu-Fe phase compositions which have the highest photocatalytic activity (see Fig. 1a). It might occur the mix-phases can enhance the electronic and optical properties of the films. Figure 3b shows the Cu-Fe composite films with a ratio of 2:2 at different annealing temperatures. No significant peak was observed at the annealed films lower than 600 °C. At 700 °C, the peaks of 30.08° and 35.52° correspond to the (220) and (211) Fig. 3c).
XPS spectra of the annealed Cu-Fe oxide composite films at 500, 600, 700, 800, and 900 °C for 1 h are shown in Fig. 4. Figure 4a shows symmetric peaks at binding energies of 932.5-934.5 eV and 953.5-954.2 eV assigned to The Cu 2p 3/2 and Cu 2p 1/2 , respectively. From the figure, the binding energy of 933.7 eV 30 which corresponds to CuFe 2 O 4 shows a strong peak after annealing at 700 °C. Whereas the smaller peaks consist of CuO (934.2 eV) 31 and CuFeO 2 (932.6 eV) 32 .
The two peaks at 710.5-712.8 eV and 724.5-726.8 eV attributed to Fe 2p 3/2 and Fe 2p 1/2 which are characteristic of Fe ions in Cu-Fe oxide films. After the film annealing at 700° C, the highest peak assigned to CuFeO 2 at 710.6 eV 33 . Moreover, two smaller peaks located at 711.2 34 and 712.8 eV 35 corresponds to γ-Fe 2 O 3 and CuFe 2 O 4 , respectively. Figure 4c shows the O 1s spectra at various annealing temperatures which can be deconvoluted into three peaks at 532.9 36 , 531.1 37 , and 530.2 eV 38 , which correspond to SiO 2 , Cu(OH) 2 , and Fe(OH)O, respectively. However, the peak of SiO 2 (532.9 eV) increased with increasing the annealing temperature. This is due to the agglomeration of network particles at a high temperature can increase the quartz substrate spacing, which corresponds to the SEM result (see Fig. 2a).
The effect of the annealing temperature on the phase transformation can be evaluated by XPS (shown in Fig. 4) are shown in Fig. 4d. It is noted that the CuFe 2 O 4 and CuFeO 2 were increased with increasing the annealing temperature 39 . However, an exceed CuFeO 2 at the annealing temperature higher than 700 °C might inhibit the photocatalytic reactions. This is due to the factors causing the thin films to have an increased energy gap when the temperature is higher than 700 °C because of the effects of % rate of the crystal structure, microstructure characteristics, and the characteristics of chemical compositions 13  www.nature.com/scientificreports/ Figure 5 shows the energy band gap (E g ) of the as-deposited, the annealed Cu-Fe oxide films at 500, 600, 700, 800 and 900 °C which are 5.35 eV, 3.88 eV, 2.89 eV, 2.56 eV, 2.94 eV and 5.63 eV, respectively. This behavior can be described by an atom distancing increased with the increasing of annealing temperature 40 . Interestingly, the annealing at 700 °C not only shows the lowest E g but also shows the highest photocatalytic activity (see Fig. 2b). This is because of the good mixing ratio between Cu and Fe oxide phases 41 .
Increasing of photocatalytic activity in the annealed Cu-Fe oxide film at 700 °C with a ratio of 2:2 can be described by Cu-Fe mixed-phase mechanism, as shown in Fig. 6. The generation of photocatalytic mechanism is based on pairs of electrons (e − ) and holes (h + ) over the composites 42     www.nature.com/scientificreports/ CuFeO 2 , which can improve the charge separation and inhibit the e − /h + recombination. This is a key factor for the enhancing photocatalytic activity of the annealed Cu-Fe oxide film at 700 °C with a ratio of 2:2 which is greater than a photocatalyst as TiO 2 .